Processes for the oligomerization of light olefins to produce C8 olefin oligomers are known. Oligomerization processes have been long employed to produce high quality motor fuel from C4 olefins. Such oligomerization processes are also referred to as catalytic condensation and polymerization with the resulting motor fuel often referred to as polymer gasoline. Methods have always been sought to improve the octane number of the gasoline boiling range oligomerization products. Indirect alkylation is a noteworthy C4 olefin dimerization process.
In one form of the indirect alkylation process, an ionic exchange resin catalyst oligomerizes light olefins to produce oligomers such as C8 olefins. In such processes, the oligomerization zone can be preceded by a dehydrogenation zone to convert paraffinic feed into olefinic feed and/or succeeded by a hydrogenation zone to convert heavy oligomeric olefins into heavy alkanes that can be blended with gasoline stock.
U.S. Pat. No. 4,313,016 B1 discloses a heat exchanged oligomerization reactor that contains a cationic exchange resin catalyst C4 olefins contacted with the resin catalyst oligomerize to C4 oligomers. Water or methanol may be present in small amounts insufficient to form an entrained second phase to serve as a catalyst modifier.
Modern oligomerization processes often include an oxygenate such as tert-butyl alcohol (TBA) and/or sec-butyl alcohol (SBA) in the feed for modifying the catalyst to maintain desired product selectivity. The modifier does not participate in the reaction. References disclosing resin catalyzed oligomerization in the presence of an oxygenate modifier include U.S. Pat. No. 5,877,372 B1 and EP 994 088 A1. TBA and SBA have become the resin catalyst modifier of preference.
More recently, higher quantities of alcohol modifier have been used in resin catalyzed oligomerizations. Consequently, removing the alcohol modifier from the hydrocarbon oligomerization product stream has become more important. In such oligomerization processes, it is typically necessary to separate unreacted light olefins from the product heavy oligomers in the effluent from the oligomerization zone. Separation is conventionally performed in a distillation column typically following the oligomerization zone. The lighter components comprising primarily unreacted C4− olefins and compounds that were present in the feed stream exit from the overhead of the distillation column. The heavier components comprising primarily heavy oligomers such as C5+ olefins and compounds exit out the bottoms of the distillation column. If the distillation column is operated to send all of the C5+ material contained in the oligomerization effluent to the bottoms, most of the alcohol modifier would exit with the bottoms product and only a small amount of alcohol modifier would exit in the overhead stream. A water wash column was designed to treat the bottoms product and recover the alcohol modifier before the C5+ stream proceeded to product storage or further treatment.
Reid vapor pressure is a standard unit used in governmental specifications regarding gasoline product vapor pressures. To meet increasingly tight governmental specifications, the distillation column must be operated so as to control the vapor pressure of the bottoms product. In this case, some of the C5 compounds are diverted from the bottoms to the overhead product. Consequently, a portion of the alcohol modifier will azeotrope with the C5 material and both the overhead and bottoms stream will contain alcohol modifier. As such, alcohol modifier must be removed from both the overhead and the bottoms product streams.
U.S. Pat. No. 4,956,513 B1 discloses an oligomerization process that uses a homogeneous boron trifluoride catalyst with a promoter such as normal butanol. After the oligomerization, the boron trifluoride catalyst is extracted from the reactor effluent by water washing. The water extract containing the major part of the boron trifluoride catalyst is then distilled to remove the water in the promoter.
U.S. Pat. No. 5,146,032 B1 discloses reacting C3+ olefins with methanol over a ZSM-5 catalyst to produce a range of hydrocarbons including an olefinic gasoline stream. The unreacted methanol and water present are separated by cooling, phase separation and, in some cases, by water washing of the hydrocarbon effluent leaving the reactor. They are then led to a methanol-water separator such as a distillation tower.
An object of the present invention is to fractionate oligomerization effluent to provide a heavy oligomer bottoms product with a desired vapor pressure and a light olefin overhead stream with a sufficiently low concentration of alcohol modifier.
An additional object of the present invention is to fractionate oligomerization effluent without having to utilize a separate water wash column on both the heavy oligomer bottoms product and the light olefin overhead streams